The electrophysiological and morphological characterisation of aminergic responsive neurones within the rat hypothalamic arcuate nucleus in vitro

Abstract

1. The hypothalamic arcuate nucleus (Arc) is a key integrative centre of the central nervous\ud
system (CNS) involved in the control and maintenance of energy balance. Whole-cell patch\ud
clamp recording techniques were utilised, in isolated hypothalamic brain slice preparations, to\ud
investigate the electrophysiological and morphological properties of Arc neurones. Differential\ud
expression of subthreshold active conductances were identified and used to functionally\ud
classify Arc neurones into 8 electrophysiological clusters. This classification was based\ud
based upon differential expression of the following conductances: anomalous inward\ud
rectification (Ian); hyperpolarisation-activated non-selective cation conductance (Ih); transient\ud
outward rectification (Ia); T-type-like calcium conductance. Morphological analysis of recorded\ud
neurones, revealed retrospectively with biocytin staining, showed four populations based upon\ud
the orientation and number of primary dendrites. There were no obvious direct correlations\ud
between morphology and electrophysiological properties, suggesting considerable functional\ud
diversity of neurones and their associated circuits at the level of the Arc.\ud
2. The physiological levels of glucose to which the brain is exposed are believed to be around\ud
1-2.5 mM, and glucose-sensing neurones have been identified in the Arc. However, in vitro\ud
slice studies routinely use glucose around 10 mM in aCSF. The impact of this high level of\ud
glucose on fundamental properties and operation of hypothalamic circuits remains unclear.\ud
Here the effect of different ambient glucose levels (10 mM, hyperglycaemic and 2 mM,\ud
euglycaemic) on electrophysiological properties of Arc neurones was compared. Significant\ud
differences in passive and active subthreshold membrane properties of Arc neurones were\ud
observed, including: changes in neuronal input resistance, spontaneous activity and\ud
magnitude of Ih and Ia. Data from this study suggests a need to re-evaluate studies previously\ud
conducted in non-physiological levels of glucose.\ud
3. The effects of noradrenaline (NA) on the neuronal excitability of hypothalamic Arc neurones\ud
were studied. Application of NA induced a membrane depolarisation and increase in electrical\ud
excitability in 51% of Arc neurones, including orexigenic NPY/AgRP neurones, a response\ud
that persisted in the presence of TTX indicating a direct effect. NA-induced depolarisation was\ud
mediated through α1-ARs, in particular through α1A-ARs, and associated with multiple ionic\ud
mechanisms including: closure of a potassium conductance, activation of a non-selective\ud
cation conductance, or a combination of the two.\ud
4. NA also induced a membrane hyperpolarisation in a sub-population of Arc neurones (15%)\ud
including 4/9 putative anorexigenic CART-expressing neurones, the remaining CART\ud
neurones responded with a NA-induced excitation. NA-induced hyperpolarisation, mediated\ud
via α2-ARs and activation of one or more potassium conductances, persisted in the presence\ud
of TTX indicating a direct effect on Arc neurones. 7.5% of neurones responded to NA with\ud
biphasic inhibitory/excitatory responses. Taken together, these data suggest that NA, at least\ud
in part, excites a subpopulation of NPY/AgRP neurones and inhibits a population of CART\ud
expressing neurones which may serve an orexigenic role at the level of the Arc.\ud
5. Histamine induced membrane depolarisation in a population of Arc neurones (65%), most\ud
likely through H1 receptors, via a direct effect on the postsysnaptic membrane. Histamine\ud
induced depolarisation through multiple ionic mechanisms, including closure of a potassium\ud
conductance or activation of an electrogenic pump